This application relates to the art of medical treatments, and, more particularly, to the application of electrical stimuli to tissues for achieving some desired result.
The invention will be described with particular reference to defibrillation of fibrillating myocardial tissues, and will be described with particular reference thereto. However, it is to be appreciated that the invention has broader applications, and is adapted to other uses and applications. One such other application, for example, is in treatment of tachyarrhythmias.
Electrical stimulation of cells in arrhythmic myocardial tissue has become a well established technique for heart defibrillation. Early waveforms for achieving defibrillation of fibrillating myocardial tissue comprised single voltage pulses in one polarity. The waveshape of these early defibrillators generally comprised a critically dampened RLC form. Though such defibrillation was often successful, it was discovered that several drawbacks were presented with attempted defibrillation through electric stimulation.
If an insufficient voltage were to be applied, the arrhythmia would not be reversed. If too great a voltage were to be applied, refibrillation and other dysfunction may occur. Early critically dampened RLC waveshape defibrillators had a relatively narrow range of voltages with which defibrillation could be accomplished without realizing a large percentage of refibrillation occurrences. A "safety factor" has been defined as the ratio between a voltage producing post-shock dysfunction and a voltage producing cellular excitation. Waveforms with large safety factors defibrillate effectively with little post-shock dysfunction, and waveforms with low safety factors have a low rate of successful defibrillation and produce much postshock dysfunction. Critically dampened RLC waveshapes have a relatively low safety factor. Another type of defibrillator implemented trapezoidal-shaped waves.
Since it has been determined that normal cells have a membrane or resting potential of approximately -90 mV, while cells in arrhythmic myocardial tissues have a resting potential of approximately -60 mV due to a number of factors, e.g., an increase of extracellular potassium and the like, it was concluded that a pre or conditioning pulse of opposite polarity, when applied to cells in arrhythmic myocardial tissues prior to the application of a defibrillating pulse, allowed reactivation of fast excitation channels in the membranes. This produces a substantially lower defibrillation threshold, correspondingly higher safety factor, and a substantially greater chance of defibrillation.
Another modification of defibrillating waveshapes comprises an initial or defibrillating pulse followed by a tail or undershoot pulse of opposite polarity. This waveshape was generally rounded or square in form and was concluded to be of advantage in decreasing the potential for post-shock dysfunction.
While methods and apparatus incorporating various defibrillation and associated pulses have been used with success, it has been considered desirable to improve upon the capabilities of and results obtained from these prior techniques. The subject invention is deemed to meet these needes and others, and provide omre efficient and reliable defibrillation method and apparatus.